Communication of digital data, especially digital video data, can be a demanding enterprise. The desire for improved performance has led investigators to attempt alternative methods and modes of communicating digital video. As a result, a wide variety of data communications technologies, including equipment, standards, protocols, etc., have been developed with varying performance characteristics.
One standard for transporting digital video data is known as SMPTE 259M, developed by the Society of Motion Picture and Television Engineers (SMPTE) for communication over coaxial cables. This standard describes a method for use with switched or dedicated interconnecting cables with one signal per cable confined to relatively short distance communication, for example within a building or campus environment.
Recent applications of the SMPTE 259M standard have indicated a need to extend its use to conditions differing from those originally contemplated. For example, users have indicated a desire to employ the standard for data communication over longer distances, over alternative physical media such as optical fibers, and without the limitation of one signal per cable. Recent improvements in fiber optic communications systems have spurred interest in combining SMPTE 259M signals with other signal types in a multi-service fiber backbone, for example a dense wavelength division multiplexed (DWDM) backbone.
One problem with communicating SMPTE 259M signals over optical communications systems involves the content of the signals and how that content affects the performance of certain optical devices employed in the system. For example, certain signals known as “pathological signals” can adversely affect the power control circuitry of optical transmitters and/or receivers and increase signal-to-noise ratio, bit error rate, intersymbol interference, and/or other adverse effects. One exemplary pathological signal includes a repeating pattern of digital bits in which one bit (either high or low) is followed by nineteen consecutive bits of the opposite polarity, i.e. 01111111111111111111 or 10000000000000000000. Such a pathological signal may include significant low frequency content and may disrupt system devices not designed for low frequency signals.
Optical transmitters commonly include power control circuitry that may attempt to keep the average optical output power at a predetermined level to compensate for degradation of the laser threshold over time and temperature. The power control circuitry responds according to a long time constant relative to the modulation waveform of the transmitted data signal, essentially using the DC portion of the modulation spectrum for laser output power control. If a transmitted data signal includes significant low frequency content, the transmitted waveform may be distorted by the power control circuitry, as illustrated in FIG. 2.
FIG. 2 is a simplified plot 200 of time versus optical output power for an exemplary optical transmitter driven with low frequency data. The optical transmitter's power control circuitry tends to increase or decrease the optical output power toward the average optical output power 206 over time, thus distorting the waveform at peak output power 204 (e.g. corresponding to long runs of high bits or ones) and at lower output power near the laser threshold 202 (e.g. corresponding to long runs of low bits or zeros). Such waveform distortions may cause the bit error rate and picture quality at the receiver to suffer.
Optical receivers and transmitters often used in optical communications systems are commonly designed under an assumption that the signal being transmitted is a 50% duty cycle modulating (e.g. AC-balanced) signal over some period of time such as 1 μs. For digital video data, an exemplary pathological signal including the repeating pattern with 1:19 ratio of ones to zeros described above may continue for up to about 50 μs, which of course fails to conform to the design assumption. During such pathological signals, the average optical output power may increase or decrease, significantly distorting the transmitted waveform and possibly causing overmodulation and/or adversely affecting signal-to-noise ratio. Overmodulation of the optical transmitter may cause additional waveform distortion and ringing, leading to intersymbol interference as the laser drive current moves into nonlinear regions of the light versus current (LI) curve such as that shown in FIG. 1.
FIG. 1 is a simplified plot 100 of laser drive current versus optical output power for an exemplary optical transmitter. As the drive current decreases from its level at peak optical output power 104 to the laser threshold 102, overmodulation can result in spectral broadening of the laser output as the laser acts more like an LED, for example when the drive current approaches the “knee” of the LI curve (e.g. near the laser threshold 102). In this way, waveform distortion can result in intersymbol interference which in turn may cause increased bit error rate, corruption of video data, and/or degradation of picture quality.
Therefore, there is a need to solve the above-described problems associated with transporting data including pathological signals over optical communications systems.